Lpc2000

Sean wrote:

>What type of efficiency does a linear regulator give you when setup in a 
>switching-style topology for low currents?
>
>  
>
Figure about 30%, maybe %40, efficiency for a linear, three terminal 
regulator.  It is not possible to use a Linear Regulator as a switched 
element...


TomW



>Check out L7805C datasheet figure 27 in case you don't know what I'm 
>talking about.
>
>While we're on offtopic, what diodes do you guys recommend using, 
>especially for lithium backup purposes?  Most diodes that I see that are 
>frequently used have large forward voltage drops for any non-trivial 
>current draw.
>
>-- Sean
>
>At 12:50 PM 12/2/2005, Guillermo Prandi wrote:
>  
>
>>Thanks, Joel. Efficiency on those chips seems still very low (around
>>50% for my circuit's average load, 15 mA), though the low quiescent
>>current seems tempting.
>>
>>Guille
>>
>>--- In lpc2000@yahoogroups.com, "Joel Winarske" <joelw@i...> wrote:
>>    
>>
>>>Guille wrote,
>>>
>>>      
>>>
>>>>Hi! I am in trouble trying to find a voltage regulator suitable
>>>>        
>>>>
>>for my
>>    
>>
>>>>project; my project features an LPC2138, a GPS and a radiomodem.
>>>>        
>>>>
>>When
>>    
>>
>>>>idle, it draws around 20-25 mA from the 3.3V source, 500 mA when
>>>>working hard and with short (50 mS) peaks of up to 1.8A. The
>>>>        
>>>>
>>supply is
>>    
>>
>>>>a lead-acid 12V battery (can't change that), and occasionally,
>>>>        
>>>>
>>when the
>>    
>>
>>>>battery is charging, the input voltage will rise up to ~20V. The
>>>>problem is the regulator efficiency. We were going for the LM2676-
>>>>        
>>>>
>>3.3V
>>    
>>
>>>>switching regulator, because it seemed simple and in range, but
>>>>        
>>>>
>>two
>>    
>>
>>>>things made it a poor choice: it draws about 4.3 mA from the
>>>>        
>>>>
>>battery
>>    
>>
>>>>just for its own supply, and its efficiency when the load is in
>>>>        
>>>>
>>the 20-
>>    
>>
>>>>25 mA range is awful (less than 50%, not counting Iquiescent). I
>>>>        
>>>>
>>looked
>>    
>>
>>>>everywhere but I couldn't find any other switcher with better
>>>>performance in these conditions (12V in, 3.3V out, 20-25 mA
>>>>        
>>>>
>>average
>>    
>>
>>>>load, but >1.5A capability). And there's a catch! The radiomodem
>>>>        
>>>>
>>is
>>    
>>
>>>>extremely sensitive to VCC getting lower than 3.3V especially in
>>>>        
>>>>
>>the
>>    
>>
>>>>current peaks! Also, it should be relatively easy to find in ~400
>>>>quantities (i.e., Digikey, Mouser or your suggestion).
>>>>        
>>>>
>>>I've used the MAX5035 for automotive apps.  Nice part.
>>><http://www.maxim->http://www.maxim-
>>>      
>>>
>>ic.com/solutions/automotive/parts.mvp/scpk/1458/pl_pk/0
>>    
>>
>>>You might consider the MAX5090, it goes up to 2A.  The input
>>>      
>>>
>>voltage range
>>    
>>
>>>is 6.5V to 72V.
>>>
>>>Joel
>>>
>>>      
>>>
>>
>>
>>
>>
>>
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-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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